The development and use of laser systems in surgery and other medical applications continues to expand at an ever increasing rate as new technology becomes available and new applications for lasers are discovered. Currently used surgical laser systems include the CO.sub.2 laser device which produces a light beam having a wavelength of 10.6 microns, and solid-state devices such as the Nd:YAG laser (Neodymium Yttrium Aluminum Garnet, Y.sub.3 Al.sub.5 O.sub.12) laser, which produces a light beam having a wavelength of approximately 1.064 microns, the argon-ion laser producing a light beam having a wavelength of approximately 0.5145 microns (or 514.5 nm), the Erbium:YAG (Er:YAG) laser producing a light beam having a wavelength of approximately 2.9 microns, the Holmium:YAG (Ho:YAG) laser producing a light beam having a wavelength of approximately 2.1 microns, and, more recently, a frequency-doubled Nd YAG laser producing a light beam having a wavelength of approximately 0.532 microns (532 nm). Heretofore, however, the Er:YAG and Ho:YAG lasers have been being used only in experimental applications, as they have yet to obtain universal approval for clinical use.
It has been found that because human tissue is approximately 80% water, the absorption of radiation energy (i.e. light energy) in water will determine the characteristics of laser interaction in tissue. The CO.sub.2 laser has been found to provide a very good "light knife" due to its ability to induce incisions with less charring with good hemostatic control; however, the Nd:YAG laser has better photocoagulative ability, as its 1.064 micron wavelength penetrates much deeper into tissue than the 10.6 micron radiation, and is closer to the hemoglobin absorption peak (i.e. approximately 0.577 microns). Because the water absorption peak has been found to be approximately 2.9 microns, the Er:YAG laser is of special interest as providing an optimum "light knife" whose light beam wavelength is much closer to the absorption peak of hemoglobin (i.e. blood), and should theoretically provide better coagulative effects in conjunction with its superb cutting abilities. In practice, however, it has been observed that Er:YAG radiation is absorbed so strongly by the water content of the tissue that it provides very poor hemostasis.
On the other hand, it has been established by theory and experiment that a relationship exists between the time which tissue is exposed to light beam energy and the size of the surrounding zone of thermal damage caused by that light beam. It has been found that rapid, short "bursts" or "pulses" of laser light can help to minimize the surrounding zone of thermal damage caused by laser cutting. Because the Er:YAG laser technology is relatively new and immature, and because of the relatively longer wavelength of its output, efficient technology capable of providing short pulses of the output radiation is not available. In contrast, the relatively well developed technology of the Nd:YAG laser can provide electro-optical pulsing (Q-switched technology) which can be up to 100 times shorter than the relatively crude pulsing technology currently available for Er:YAG lasers. Similarly, reliable pulsing technology has yet to be developed for Ho:YAG lasers.
Consequently, while the Er:YAG and Ho:YAG lasers provide laser radiation at wavelengths much closer to the absorption peak of water, the inability to precisely control the temporal application of such laser radiation tends to result in increased thermal diffusion beneath the laser excision, which can result in increased inflammatory response within the tissue, delaying healing and increasing the chances of post operative infection. Another laser delivery system known as the HF laser can also produce laser radiation at a wavelength of approximately 2.9 microns, however, the use of the HF laser in medical applications is felt by many to be inappropriate because of the large size of the device and the use flowing SF.sub.6 --a toxic gas--to produce free fluorine).
The 1.064 micron wavelength output of the Nd:YAG laser provides better coagulative features as a result of relatively deeper penetration into the tissue and resultant enhanced hemostatic control. On the other hand, a light beam at the 1.064 micron wavelength creates inferior incisions as increased charring of surrounding tissue is created. In fact, as the wavelength of a particular laser light beam is decreased toward the Hemoglobin absorption peak (i.e. between approximately 0.5 and 0.6 microns), the ability for hemostatic control increases, while the precision or "cleanness" of the incision decreases. While a particular laser can sometimes be chosen for optimum surgical conditions (such as in cornea surgery where there is no bleeding and optimal incision control can be obtained by utilizing a laser which produces radiation at a wavelength within the peak of the water absorption spectrum of 2.85 to 2.95 microns), too often a tradeoff must be made between the surgeon's desire to obtain the most precise and clean incision, and a desire to minimize thermal damage and to optimize hemostasis. Additionally, due to the relatively high cost of laser equipment, rarely does a physician have the luxury of choosing between several types of laser devices for any particular surgical procedure.
Because a variety of solid-state laser devices are available in the industry which provide laser radiation at wavelengths in the relatively longer ranges of the spectrum (i.e. 0.700-3.0 microns), it has not been uncommon to utilize available technology for doubling the frequency of the output of one of these devices to reduce the wavelength to the visible spectrum and/or to provide laser energy closer to the absorption peak of hemoglobin for increased hemostatic control. U.S. Pat. Nos. 4,639,923, 4,739,507, and 4,809,291 are examples of devices which provide for doubling of frequencies to reduce the wavelength of laser radiation provided by a particular laser device. While frequency doubling can reduce the resultant power provided by any particular laser device by 30% or more, this procedure often represents the only practical way of obtaining laser radiation to provide for visible laser light and/or increased hemostatic control.
For example, laser radiation from the Nd:YAG laser can be frequency doubled by utilization of well-known and relatively readily available KTP (KTiOPO.sub.4) crystals or Beta-Barium-Borate (B--BaB.sub.2 O.sub.4 or BBO) crystals to provide laser radiation at a wavelength of 0.532 microns. While laser radiation at a wavelength of approximately 0.577 microns can be provided with a dye laser system, this wavelength cannot be produced by currently available solid-state systems, which are much preferred for surgical application due to their reliability and ease of use and maintenance.
Optical parametric oscillator (OPO) technology has also been utilized to convert laser radiation to longer wavelengths in a more reliable solid-state form. U.S. Pat. No. 4,180,751 includes a description of the utilization of an OPO device to provide a signal or idler frequency from a pump wavelength. OPO is the inverse of sum-frequency generation processes like second harmonic generation. In OPO conversion, two variable frequencies, related as follows: ##EQU1## where .lambda.p is the pump wavelength, are determined by the particular phase matching used. Only one pair of frequencies can be phase matched at a time. By adjusting the phase matching parameters, e.g., the temperature or orientation of the non-linear crystal in an OPO setup, the output can be "tuned" over a range of frequencies. In OPO arrangements, the pump wavelength is always converted into two longer wavelength components, .lambda.1 and .lambda.2.
Other efforts have been directed to providing laser light at a single optimal wavelength which could provide satisfactory cutting and coagulative abilities. In fact, the development of the Ho:YAG laser with its 2.1 micron output is understood to have been the result of just such a study. No such optimal single wavelength has been identified, however, as results generally show inferior cutting and inferior sealing.
It has been speculated that a compound laser system capable of producing the cleanness of incision of a CO.sub.2 laser or an Er:YAG laser, along with the photocoagulative ability and hemostatic control of, for example, a frequency doubled Nd:YAG laser would be a valuable surgical tool. As set forth in his article titled "Laser Surgery: CO.sub.2 or HF", Myron L. Wolbarsht (IEEE J. Quantum Electronics, QE-20, No. 12, December, 1984), such a compound laser system was envisioned as literally incorporating several laser types available for simultaneous use. Wolbarsht specifically suggested the use of HF laser technology for optimum cutting, and an argon-ion or Nd:YAG laser for deeper penetration and increased coagulation. As set forth above, the size and toxic nature of the gas of the HF device makes it a poor match for use in surgical applications, and the combination of two expensive laser devices would not only be cost prohibitive, but would also be difficult to structurally arrange for convenient and accurate use. Moreover, the development of fiber optic delivery systems for the CO.sub.2 wavelength laser continues to lag far behind those for other medically useful wavelengths such as the 1.064 micron YAG laser. In any event, no such device has been made available in the industry.
Consequently, heretofore there has not been provided a single practical device for delivering a single laser light beam having two or more medically useable wavelengths which could, for example, optimize simultaneous cutting and sealing in a single operation. Moreover, there has not been available a solid-state laser which can simultaneously develop two medically desirable wavelengths (such as 0.532 microns and 2.9 microns) for instantaneously producing medically desired results, such as superior incisions and optimum coagulative ability, with a single tool.